The present invention relates generally to a smoke-monitoring system for use in measuring the opacity of exhaust from an internal combustion engine, and in particular, to an unpowered system that maintains the exposed optical portions of the opacity monitor free of particulate contaminants.
In analyzing the performance of an internal combustion engine, it is often desirable and sometimes essential that the smoke content of the exhaust from the engine be measured and analyzed. This task is usually accomplished by using an opacity monitor that includes a light source and sensor mounted on opposite sides of the engine's exhaust pipe such that an optical path extends between the light source and the sensor across the exhaust stream. In order to protect the sensitive optics and electronics of the light source and sensor, these two portions of the opacity monitor are normally maintained in separate sealed compartments mounted on either side of the exhaust pipe of the engine. Each compartment includes a transparent window at one end such that the optical path extending between the light source and the sensor passes through the two windows. While the windows serve to protect the sensitive optical and electronic elements of the monitoring system, the transparency of the windows must be maintained in order to preserve the accuracy of the monitoring system. In other words, some means must be provided that prevents the build-up of particulate matter on the exposed faces of the windows.
In order to overcome this problem, many opacity monitors include a blower system that continuously blows particulate-free air across the exposed surface of the windows in order to prevent the exhaust smoke and other potential contaminants from coming into contact with the windows. This is accomplished by mounting the windows within air ducts that have inlets that open outside of the exhaust pipe and outlets which open into the exhaust pipe. Blowers mounted in the air ducts draw fresh air into the ducts through filters to continuously provide particulate-free air flow across the exposed faces of the windows. While the blower-type system may be acceptable in some applications, what is needed is an unpowered self-cleaning system that automatically maintains the cleanliness of the windows without requiring powered blowers or access to a compressed air supply.